A simple cryogenic refrigerator can be provided by a pair of compressors that move the working fluid cyclically in opposite directions through a Joule Thomson expansion valve. U.S. Pat. No. 4,366,680 by Tward describes a system of this type, wherein a gaseous working fluid at a first temperature moves through stages of progressive cooling. These stages include a precooler that cools the fluid to a second lower temperature, a heat exchanger that cools the fluid to a third lower temperature, an expansion chamber that cools the fluid to a fourth lower temperature, and a Joule Thomson expansion valve that cools the fluid to a lower fifth temperature. Fluid on the downstream side of the valve is coupled through a heat switch to the thermal load that is to be cooled. The fluid continues to a second compressor which initially takes up the fluid and later moves it in a reverse direction through several stages of cooling, until the fluid passes in the reverse direction through the expansion valve. Fluid at the new downstream side of the valve is coupled through another heat switch to the thermal load. The use of heat switches operating at very low temperature wastes cooling capacity and adds complexity to the system. A system which avoided the need for heat switches at the coldest temperature, would be of considerable value.
When a Joule Thomson refrigeration system first starts operating, the working fluid is warm, and only after a considerable period of time does the working fluid achieve steady state operation when the fluid achieves a minimum temperature at each location of the system. For almost all working fluids, the density of the fluid increases as it becomes colder. Where the Joule Thomson valve is set for optimum operation at steady state condition when the fluid is cold and dense, the system will operate inefficiently during startup when the fluid is warmer and less dense. For example, common working fluids such as helium may undergo a change in density of 15 to 1 between room temperature and a cryogenic temperature such as 20.degree. K. An expansion valve whose resistance to fluid flow therethrough is optimal for steady state condition is at 20.degree. K., will permit only a very low mass flow rate of fluid therethrough during startup. This greatly increases the period between initial startup of the system and achievement of a desired low temperature. A system which more efficiently operated during startup would be of considerable value.